Purpose:The purpose of this study was to assess and compare wettability, dimensional changes, flexibility and tear resistance of two recent elastomeric impression materials, vinyl polyether silicone (VPES) hybrid material and polyvinyl siloxane (PVS) containing nanofillers with conventional PVS and polyether (PE) impression materials.Materials and methods:The elastomeric impression materials are divided into four groups, two experimental: VPES (EXA'lence) and PVS containing nanofillers (Elite HD+) and two control groups: PVS (Imprint II) and PE (Impregum Soft). The properties that were investigated are wettability, dimensional changes%, flexibility and tear resistance. Ten specimens were made for each property of all groups.Results: The results indicated that VPES (EXA'lence) and PE (Impregum Soft) showed the highest wettability. PVS containing nanofillers (Elite HD+) showed the lowest dimensional changes%. The strain in compression % (flexibility) of PE, VPES and PVS were not significantly different from each other and they were significantly different from PVS containing nanofillers which showed the lowest strain in compression%. The tear resistance (N/mm) of PVS containing nanofillers (Elite HD+) and VPES (EXA'lence) were significantly higher than PE (Impregum Soft) and PVS (Imprint II) which were not significantly different from each other.Conclusion: All tested materials were hydrophilic specially VPES and PE which recorded the highest wettability. All tested mterials showed accepted dimensional changes according to ANSI/ADA No. 19 and PVS containing nanofillers showed the greatest dimensional stability. All materials recorded accepted flexibility according to ANSI/ADA No. 19. As related to flexibility; PE, VPES and PVS recorded higher flexibility than PVS containing nanofillers while PVS containing nanofillers and VPES showed the highest tear resistance.

Impression is the most important procedural step in dentistry which will impress not only the professionals but also the patient by accurate replacement of dento-orofacial structures [1]. In the last decade, several investigators have recommended using newer elastomeric materials such as polyvinyl siloxane (PVS) and polyether (PE) for final impressions to replace the older and more traditional materials [2],[3].

Contemporary impression materials have a number of properties that contribute to clinical success. The hydrophilicity of the impression materials is critically crucial to wet the hard and soft-tissues in the mouth and to create accurate impressions and casts [4]. A material exhibiting contact angle of greater than 90° is an indication of poor wetting and hydrophobicity; while a material exhibiting contact angle of less than 90° are an indication of better wetting and hydrophilicity [5]. PVS are inherently hydrophobic. However 'hydrophilic' PVS have been introduced with manufacturer claims that they better wet moist dental surfaces. These formulations have surfactants added [6]. This mechanism differs from PEs, which possess a high degree of wettability because their molecular structure contains polar oxygen atoms, which have an affinity for water [7]. In dentistry, accurate and dimensionally stable impressions are the first step toward fabrication of a successful prosthesis [8]. PVSs show the smallest dimensional changes on setting of all the elastomeric impression materials [9].

An impression material that is excessively rigid hinders its removal over tissue undercuts and also increases the likelihood of die breakage upon its removal from the stone die [10]. The strain in compression% is a measure of the flexibility of the material. In general, the low-consistency materials of each type are more flexible than the high-consistency elastomeric impressions [7]. The conventional belief is that for a given consistency, PEs are generally the stiffest, followed by addition silicones [7],[11]. A potentially clinically useful change has been the production of a soft PE with reduced stiffness when compared with the traditional material [12].

Impressions should resist tearing when tensile stresses are applied during impression removal and cast separation from the set impression. Impression materials are most susceptible to tearing in gingival crevices and interproximal areas. Tearing in the impression causes defects, which affect the accuracy of the final restoration [13].

In 2009, a vinyl polyether silicone (VPES) product (EXA'lence; GC Europe NV, Leuven, Belgium) was commercially introduced. This material is available in a variety of consistencies and setting times. EXA'lence is composed of a combination of PVS and PE and is promoted as a hydrophilic material that presumably maintains the stability of the parent products [14]. The chemistry of the material is new and allows for the material to be intrinsically hydrophilic, without the need for the built in surfactants like traditionally PVS materials. The manufacturer claims that the material incorporates high tear strength properties, improved elasticity and marginal accuracy [15].

Recently, silica nanofillers are integrated in PVS, producing a unique addition of siloxane impression materials. The nanofillers are processed by top–down approach. Elite HD+ are available impression materials which use nanotechnology. The material has better flow, improved hydrophilic properties, and enhanced detail precision [16]. The manufacturer claimed that insertion of the nanometric particles and the other components of the material create a nanostructure in a configuration which enables a degree of fluidity completely different from the initial viscosity to be obtained. When pressure is exerted in taking the impression, an excellent reproduction of infinitely small details is obtained.

The aim of this study was to assess and compare wettability, dimensional changes, flexibility and tear resistance of two recent elastomeric impression materials (VPES and PVS containing nanofillers) with PVS and PE.

The null hypothesis of this study was that VPES (EXA'lence) and addition silicone containing nanofillers (Elite HD+) would not show a significant difference in wettability, dimensional changes, flexibility and tear resistance compared with PVS and PE impression materials.

Materials and Methods

The elastomeric impression materials used in this study are presented in [Table 1].

The elastomeric impression materials are divided into four groups, two experimental and two control groups:

Experimental groups:

Group 1: VPES material (combination of PVS and PE) (EXA'lence)

Group 2: PVS containing nanofillers (Elite HD+).

Control groups:

Group 3: PVS (Imprint II)

Group 4: PE (Impregum Soft).

The properties that were investigated are wettability, dimensional changes%, flexibility and tear resistance. Ten specimens were made for each property of all groups.

Wettability

Each specimen was made using a stainless steel mold with 20 mm length, 10 mm width and 2 mm thickness [17][Figure 1]. Static sessile drop method was used to measure the advanced contact angle [18]. Controlled (0.1 ml) volume droplet of distilled water was placed onto specimen surface by means of a micropipette (Eppendorf Reference, adjustable volume; Hamburg, Germany). Digital microscope (Scope Capture Digital Microscope; Guangdong, China) with high resolution camera was used for imaging the shape of a water drop on the impression material sample surface. The captured image was analyzed by using ImageJ software (ImageJ; USA) to determine the contact angle.

Specimen preparation and testing procedures were performed according to ANSI/ADA No. 19 for elastomeric impression materials [19]. Standardized stainless steel die was used [Figure 2] for preparation of specimens. Impression material was mixed and then loaded into the mold. The mold was immediately covered with a thin polyethylene sheet followed by a rigid flat metal plate. The assembly was immediately transferred to a water bath at 32 ± 2°C. The impression was pressed out of the mold using the riser. A universal measuring microscope (Carl Zeiss, Jena, Germany), having a micrometer stage with an accuracy of 0.001 mm, was used. The distance between the cross-lines on the ruled metal block, was measured to the nearest 0.001 mm and recorded as reading A. Twenty-four hours after the impression was prepared, the distance between the cross-lines, which was reproduced in the impression, was measured and recorded as reading B. The measurement was performed each time from the inner profiles of the horizontal lines. Three measurements were made each time and the average of them was reported to the nearest 0.001 mm. Dimensional changes were calculated as follows:

The specimens were prepared according to ANSI/ADA No. 19 for elastomeric impression materials [19]. Sufficient quantity of material was mixed and then loaded into the mold. A sheet of polyethylene followed by a square metal plate was brought in contact with each end of the mold using a clamp, thus exuding excess material from the mold. The mold and the accompanied plate [Figure 3] were immersed in a 32 ± 2°C water bath.

Test procedures were performed according to ANSI/ADA No. 19 for elastomeric impression materials [19]. Six minutes after removal from the water bath, the specimen was subjected to a load on a Universal Testing Machine (Model LRX-Plus; Lloyd Instruments Ltd, Fareham, UK) calculated to produce a stress of 100 g/cm 2, the load was maintained for thirty seconds. Sixty seconds after application of a stress of 100 g/cm 2, an additional load calculated to produce a total stress on the specimen of 1000 g/cm 2 was gradually applied during an interval of 10 s and the load was maintained for 30 s. The change in length (ΔL) was measured and strain in compression was calculated as follows:

While L is the original length and ΔL is the change in length. The height of the mold (20.0 mm) was considered to be the original length of the specimen.

Tear resistance

The specimens were prepared using an ASTM standard Die C tear specimen according to ASTM Test D624 [20]. The thickness of specimen was 1.8 mm using dental flasking method. Each impression material was mixed according to the manufacturer's instructions and placed in the mold. Half an hour later the specimen was removed from the mold, the flash was trimmed, and the specimen was incubated at 23 ± 2.0°C and 95 ± 5% relative humidity for approximately half an hour. The tear test was performed according to ANSI/ADA No. 20 for dental duplicating materials [21]. One hour after a specimen was prepared; it was placed on a Universal Testing Machine (Model LRX-plus; Lloyd Instruments Ltd.) and subjected to tension (F) at a uniform rate of 25.4 cm/min until rupture occurred [Figure 4]. Tear resistance was calculated by the following equation:

Where T is the tear resistance, F is the force required to tear the specimen and D is the thickness of the specimen.

Statistical analysis

Data were collected and statistical analysis was performed using GraphPad Prism4 statistics software (GraphPad Inc., San Diego, California, USA) for Windows. As data passed normality test so one way analysis of variance (ANOVA) was used to determine the significance between groups. Newman–Keuls post-hoc test was used for pair-wise comparisons. The significance level was set at P value less than or equal to 0.05.

Results

For wettability test (contact angle measurement), one way ANOVA test revealed a highly significance difference between groups (P < 0.0001) [Table 2]. Pair-wise Newman–Keuls post-hoc test showed that the contact angle (deg.) of VPES and PE were not significantly different (P > 0.05), while they were significantly lower than that of PVS and PVS containing nanofillers (P < 0.05) which were not significantly different from each other (P > 0.05).

Considering dimensional changes% test, one way ANOVA test revealed a highly significance difference between groups (P < 0.0001) [Table 3]. Newman–Keuls post-hoc test showed that the dimensional changes% of PVS containing nanofillers was significantly lower than that of PVS (P < 0.05). The dimensional changes% of PVS and PVS containing nanofillers were significantly lower (P < 0.05) than that of PE and VPES which were not significantly different from each other (P > 0.05).

For flexibility (strain in compression%) test, the difference between groups was statistically significant as indicated by ANOVA test (P < 0.0079) ([Table 4]). Pair-wise Newman–Keuls post-hoc test showed that the strain in compression% of PE, VPES and PVS were not significantly different from each other (P > 0.05) and they were significantly different from PVS containing nanofillers which showed the lowest strain in compression% (P < 0.05).

Considering tear resistance test, the difference between groups was statistically significant as indicated by ANOVA test (P < 0.0001) [Table 5]. Pair-wise Newman–Keuls post-hoc test showed that the tear resistance (N/mm) of PVS containing nanofillers and VPES were not significantly different (P > 0.05), while they were significantly higher than PE and PVS (P < 0.05) which were not significantly different from each other (P > 0.05) ([Figure 5], [Figure 6],[Figure 7],[Figure 8].

In this study, the advanced contact angle was measured using the static sessile drop method [18]. Dimensional changes% and strain in compression% were measured according to ANSI/ADA No. 19 for elastomeric impression materials [19]. Tear resistance was measured according to ADA No. 20 which describes the tear test for nonaqueous dental duplicating material [21] and specifies the use of an ASTM [20] standard Die C tear specimen.

The results of this study showed that the combination of PVS and PE in VPES (EXA'lence) impression material as the manufacturer reported that it has 5–20% PE compound, significantly lowered the contact angle than that of PVS (Imprint II) and PVS containing nanofillers (Elite HD+). There were no significant difference between VPES and PE which had significantly lower contact angle than PVS and PVS containing nanofillers. The low contact angle of PE can be attributed to the inherent hydrophilicity of the material, originating in the chemical composition, which exhibits oxygen groups between the more hydrophobic parts. These polar oxygen atoms have an affinity for water, as stated by Craig and Powers [22]. Michalakis et al.[5] compared the hydrophilicity of six elastomeric impression materials and concluded that PE was the material that exhibited the lowest contact angle values.

PVS (Imprint II) and PVS containing nanofillers (Elite HD+) used in this study were hydrophilic by incorporation of the surfactants that change the surface properties at the solid liquid interface. Mechanism of action is probably involves surfactant solution in the wetting liquid that lowers the surface tension of the liquid and increases the surface energy of the elastomers to promote wetting property [23]. These findings were in agreement with the findings of Norling and Reisbick [24] which concluded that incorporation of surfactants into impression materials increased the wettability and produced voids free casts.

ADA No. 19 criteria state that elastomeric impression materials should not display more than 0.5% dimensional change after 24 h of polymerization of the material [19]. All materials used in this study were below these standards Chen et al.[25] stated that higher filler component may increase the accuracy. The smaller filler particles tend to aggregate among each other and contribute to reinforcement [26]. The results supported that the addition of nanofillers in Elite HD+ has increased its dimensional stability. PVS materials (Elite HD+ and Imprint II) possess a set reaction by the terminal group vinyl with hydride groups, without the formation of byproducts and with the least occurring impression material shrinkage, allowing that these materials stay dimensionally stable after impression removal [26]. As VPES contains PE group, there could be absorption of water and leaching out of water soluble plasticizers; thus leading to further shrinkage of impressions. This is in agreement with Pandita et al.[1].

Higher strain in compression values indicates more flexibility. [7] All materials in this study recorded accepted flexibility within the recommended range (2–20%) according to ANSI/ADA No. 19. Lu et al.[27] showed that there are differences in the mechanical properties of the impression materials correlated to their consistencies. The flexible materials would be expected to have less cross-linking, less fillers, or more plasticizer. The incorporation of nanofillers in Elite HD+ may be responsible for decreased flexibility. The results of this study are contrary to the conventional belief that PE is the stiffest elastomeric impression material [7],[11]. The manufacturer has made efforts to overcome the disadvantages in the most recently developed PE (Impregum Soft) by decreasing the filler ratio to render a less rigid impression. Another approach to reduce the stiffness of the polymerized material is by adding low-viscosity softeners. This in agreement with Lu et al.[27] who concluded that the PE (Impregum Soft) impression materials tested had significantly higher strain in compression compared to new addition silicone materials. This is also in accordance with Walker et al.[28] who studied the rigidity of medium-body PE, PVS, and hybrid VPES impression materials. The results showed that medium-body materials, PVS exhibited significantly higher rigidity and hardness than VPES or PE.

It was found that the tear resistance (N/mm) of PVS containing nanofillers and VPES were not significantly different, while they were significantly higher than PE and PVS which were not significantly different from each other. This is in agreement with Hondrum et al.[10] who concluded that PVS and PE were not significantly different from each other. While the results are in disagreement with Lawson et al.[29] who found that PVS showed higher tear strength than PE and a hybrid material. The reason for this difference may be as they used different test method, condition, setting time and tearing rates. The aggregated nanofillers in Elite HD+ has reinforced it and increased its tear resistance as the manufacture claimed. The manufacturer of VPES impression material has reported goal in adding PVS to PE as the material took the advantages of PVS and recorded high tear resistance.

Conclusion

Contact angle measurement showed that all tested materials were hydrophilic specially VPES and PE which recorded the highest wettability

Petrie CS, Walker MP, Williams K. A surveys of US prosthodontists and dental schools on the current materials and methods for final impressions for complete denture prosthodontics. J Prosthodont 2005; 14:253–262. [PUBMED]